1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device which has an improved barrier metal layer incorporated in an electrode or a wiring layer. It also relates to a semiconductor device which has electrodes or wiring layers of polycide structure, each comprising a polysilicon layer and a refractory metal silicide film deposited on the polysilicon layer.
2. Description of the Related Art
Hitherto, a barrier metal layer has been interposed between one wiring layer and another wiring layer or a semiconductor device when the one wiring layer is electrically contacts the other wiring layer or the semiconductor element. The barrier metal layer prevents the wiring layer from reacting the semiconductor element or the other wiring layer, or prevents diffusion of metal element between the wiring layer and the semiconductor element or the other wiring layer through the interface between the two components. Thus, the barrier metal layer not only achieves good and reliable contact between the two components. But also is it interposed between an insulating film and a wiring layer or an electrode when the wiring layer or the electrode is formed on the insulating film.
At present TiN, TiW and the like are used as materials for barrier metal layers. Films of these materials are formed by means of sputtering or the like. The films thus formed are polycrystalline, made of columnar crystals having crystal grain boundaries which extend at right angles to the surface of the underlying film. The crystal grain boundaries are likely to cause the metal to diffuse in the very direction in which the metal should not diffuse. Obviously, the films have a structure which fails to function as a barrier against the diffusion of the metal. It is desired that the wiring layer have a low resistance in order to enable the semiconductor element work with high efficiency. To this end, the barrier metal layer must be thin enough to be sufficiently low in resistance. Here arises a problem. A thin barrier metal layer acts less effectively as a barrier than a thick one. The conventional barrier metal layer does not seem to function well as a barrier. It would be necessary to use a single-crystal thin film instead, since the film can effectively work as a barrier. With the method available at present, however, it is extremely difficult to form a flawless, single-crystal thin film.
Polysilicon has been used as material for gate electrodes. Since polysilicon has high resistance, a semiconductor element having a gate electrode made of polysilicon has a high parasitic resistance. The high parasitic resistance deteriorates the characteristics of the semiconductor element. For that reason, it is proposed that metal or metal silicide, which has low resistance, be used as material for gate electrodes. When a metal film is formed on a gate insulating film by ordinary sputtering technique, however, it becomes polycrystalline and has various crystal faces. Different crystal faces have different work functions which brings about unstable difference in work functions between the metal and the semiconductor beneath the gate insulating film. The difference in work functions affect the semiconductor beneath the gate insulating film. Consequently, the semiconductor element has but an unstable threshold voltage and cannot be practically used at all.
Assume a tungsten (W) film is used as a gate electrode of a transistor. The tungsten film exhibits work functions of 5.25 eV, 4.63 eV and 4.47 eV in crystal axes (110), (100) and (111), respectively. It is required that the crystals be oriented in the same axis at the bottom of the tungsten film, which contacts a gate insulating film. Otherwise, the transistor could not have its threshold voltage controlled well.
Recently a so-called "polycide structure" has come to be used generally as a gate structure. It comprises a polysilicon layer and a refractory metal silicide layer deposited on the polysilicon layer. The metal silicide layer is made of MoSi.sub.x, WSi.sub.x or the like which has a lower electric resistance than polysilicon and a relatively high heat resistance Refractory metal silicide is an excellent material for the following reason. That is, it can be well processed applied to the process in which polysilicon is processed. The process need not be modified so much even if a polycide structure is employed.
When a polysilicon layer is processed to a shape of a gate electrode, and is oxidized, the edge portion of the polysilicon layer is oxidized to a large thickness. A transistor having a gate electrode thus formed has a high gate breakdown voltage and reliably operates for a long time, as is known in the art. Even when a refractory metal silicide film is also oxidized at its surface at the same time the surface of the polysilicon layer is oxidized, metal oxide will not be formed, but only SiO.sub.2 will be formed on the metal silicide film, if the metal silicide film contains more silicon than in stoichiometric composition.
If the metal silicide film contains more silicon than in stoichiometric composition, the silicon in the metal silicide will be consumed in oxidation. As a result, the mental content in the film will be more excessive. This composition change is prominent in inverse proportion to the width of the polycide structure. SiO.sub.2 films, each 85 nm thick, were formed on WSi.sub.x films of two types which had the same thickness of 300 nm and different initial compositions of WSi.sub.2.50 and WSix.sub.2.65, thereby forming strips of polycide structure. The average composition of the WSi.sub.x films of either type were plotted against the widths of the strips. The results were as illustrated in FIG. 1.
As is evident from FIG. 1, when the strip of either polycide structure had a width of 0.8 nm or less, the composition of either tungsten silicide (WSi.sub.x) approached the stoichiometric composition, i.e., WSi.sub.2, and the tungsten content became excessive. This is because, the narrower the strip of polycide structure, the higher the ratio of the surface area (i.e., upper and side surfaces combined) to the unit volume of the strip and, hence, the larger the amount of silicon consumed in the oxidation.
When silicon is oxidized in excess, silicon is supplied from the polysilicon layer (i.e,layer, the lower layer) into the tungsten silicide film (i.e., the upper layer) to preserve the stoichiometric composition of tungsten silicide film. Tungsten silicide (i.e., refractory metal silicide) inevitably bites into the polysilicon layer, changing the gate breakdown voltage of the transistor very much as shown in FIG. 2. More precisely, after the polysilicon layer has been thermally oxidized to form an SiO.sub.2 film, the gate current-to-gate voltage characteristic greatly of the transistor shifts from the normal relation indicated by the broken curve to one indicated by the solid-line curve.
As described above, in the conventional semiconductor device, the barrier metal layer incorporated in an electrode or wiring layer of a semiconductor element cannot acts as an effective barrier. Consequently, the characteristics of the semiconductor element and the reliability of the wiring layer are deteriorated. Further, the metal layer can hardly be used as gate electrode since work function of the metal electrode on the gate insulating film can not be controlled.
In the polycide structure used in a semiconductor element, silicon is supplied from the polysilicon layer into the refractory metal silicide film formed on the polysilicon layer, when silicon in the metal silicate film is oxidized in an excessive amount. The silicon supplied into the metal silicide film deteriorates the gate breakdown voltage of the semiconductor element. This phenomenon is prominent if the polycide structure is so narrow that far more silicon is consumed in the lateral edges of the structure than in the other portion thereof. The deterioration of breakdown voltage does not take place in the entire gate electrode (i.e., the polycide structure). It is attributed to the metal silicide locally invaded into the polysilicon layer. In other words, the gate breakdown voltage of the element is deteriorated due to the non-uniform refractory metal silicide and the fast diffusion of silicon through crystal grain boundaries.